[docs] Docker run example with privileged port

[gesellix]

What did you do?

I read the migration guide :)

What did you expect to see?

Hints about the migration from Prometheus 1.x to 2.x.

What did you see instead? Under which circumstances?

Not instead, but I stumbled about the notes about non-root users in combination with privileged ports at the end of the migration guide: https://github.com/prometheus/prometheus/blob/master/docs/migration.md#prometheus-non-root-user

The guide gives an example how to use Docker to make Prometheus available on port 80:

docker run -u root -p 80:80 prom/prometheus:v2.0.0-rc.2  --web.listen-address :80

An example like this works, too:

docker run -p 80:9090 -v `pwd`/prometheus.yml:/prometheus.yml prom/prometheus:v2.2.1 --web.external-url=http://prometheus:80 --config.file=/prometheus.yml

Please note that I also added the necessary config file via volume bind.

The example wouldn't drive users to let Prometheus run as root and it also leverages a container's port mapping possibilities - which makes the outer and inner perspective of a container more explicit.

My question is: are there any reasons for the way the current example has been documented? If not, I can submit a PR to improve the Docker example.